Thermal batteries (TBs) are primary electrochemical storage devices that rely on high temperature molten salts as their electrolyte, making them electrochemically inert at ambient temperatures where the salt is in the solid state. This prevents side reactions and self-discharge during storage, which, when combined with their hermetically sealed assembly, gives TBs an extremely long shelf life. TBs are activated by heating to high temperatures at which point the electrolyte salt melts and becomes ionically conductive, allowing for discharge of the TB. The melting temperature for most TB electrolyte salts is in the range of 250-450° C. Molten salts inherently have very high conductivities and this is further enhanced by the high temperature of the battery during discharge, enabling very high-power performance for TBs.
Since the late 1970's, thermal battery technology has been centered upon the FeS2//Li—Si electrochemical system assembled using pellet technology. A sectional view of a typical FeS2//Li—Si thermal battery is shown in FIG. 1. The discharge process for this TB is shown in Equations 1 and 2 below. In addition to the cathode and anode pellets, a separator pellet of MgO impregnated with electrolyte salt is also necessary to separate the electrodes and prevent internal shorting while a heat pellet of Fe/KClO4 is included as a heat source to bring the cell to its activation temperature. The electrolyte—most commonly LiCl:KCl eutectic salt—is mixed in as a component of the cathode and separator pellets (and occasionally the anode). This system has an open circuit potential of about 2V based on only the first cathode reaction plateau (in which Li3Fe2S4 is formed) and the Li—Si transition from Li13Si4 to Li7Si3.FeS2→Li3Fe2S4→Li2-xFe1-xS2(x˜0.2)+Fe1-xS→Li2FeS2→Li2S+Fe  (1)Li13Si4→Li7Si3→Li12Si7  (2)
The process of making these TB pellets involves pressing powders of the battery materials at high pressure to form solid pellets. These pellets are subsequently stacked in sequence to produce a battery of the desired voltage. This process is very labor intensive and as such components produced with this method are quite expensive and slow to produce. In addition, the pellet process restricts the shape of the produced pieces to circular shapes for equal pressure distribution. This subsequently restricts thermal batteries to a cylindrical form factor.
Accordingly, there is a need for a method of casting thermal battery films that is not labor intensive, is inexpensive and fast to produce, can be manufactured in different form factors, and also delivers a TB that maintains high power capability. Embodiments of the present invention meet the needs stated above and are directed towards a method of fabricating cast TB films.